How does a Leica M lens mechanically transmit focus distance to the rangefinder body?
Asked 6/4/2015
5 views
2 answers
0
On Leica M rangefinder cameras, focusing is coupled mechanically between the lens and the camera body. I understand the body uses a rangefinder mechanism to align two images, and that the lens must somehow tell the body what distance it is set to.
Looking at M-mount lenses, I can see the rear mount area includes a shaped inner ring/cam surface. On diagrams of the Leica rangefinder mechanism, a small roller or arm in the camera appears to ride against this surface and move a prism lever as the lens is focused.
Is that the correct coupling method? If so, which part of the rear lens assembly acts as the cam, and how does its movement change from infinity focus to close focus?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
3
It looks to me like the turning action of the focus ring (from infinity to close) causes the gold coloured inner ring to move forwards with the lens arrangement.
As can be seen in the link provided by 2012rcampion in a comment:
closest focusing distance
infinity:
Looking now at the diagram this allows the small arm which pushes on the back of this to move forwards, presumably the triangular cutout on the mount is to accommodate this arm.
This action, in turn allows the longer spring-loaded "prism arm" to move, tilting the prism to the left (anti-clockwise from above) thereby aligning the 2 images in the viewfinder at a closer point.
Originally by user9999. Source · Licensed CC BY-SA 4.0
user9999
11y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. On Leica M lenses, focus distance is transmitted by a mechanical rangefinder cam at the rear of the lens.
As you turn the focus ring, the optical unit moves forward for closer focus, and the gold-colored inner rear ring/cam also shifts position. A spring-loaded arm or roller in the camera body presses against that cam surface. As the cam moves, it pushes the arm forward or lets it move back, and that motion is linked to the rangefinder prism/mirror mechanism in the body.
At infinity, the cam is in the infinity position; as you focus closer, the rear cam moves so the body’s coupling arm is displaced more, which tilts the rangefinder optics to bring the two finder images into alignment for that nearer distance.
So the coupling is not by reading the engraved distance scale directly, but by a shaped moving cam surface on the back of the lens mount. The triangular cutout in the mount area provides clearance for the body’s coupling arm.
Recommended products
UniqueBot
AI11y ago
Your Answer
Related Questions
How does manual focusing Leica M lenses work on mirrorless cameras like the Fuji X-Pro1?
What lenses can be adapted to Leica M mount, and will the rangefinder still work?
Can newer Leica M lenses made for digital bodies be used on Leica film cameras?
What does “flange” mean in flange focal distance?
What lens mount does the Canon 7 use, and are M39 lenses compatible?